1. Field of the Invention
The present invention relates to a fixing apparatus, which is provided in an image forming apparatus, copier, printer, etc., to form images on a transfer material by using an electro photographic process, and fixes a developer on a transfer material to the transfer material.
2. Description of the Related Art
In a copier or printer using an electronic process, it is known that a toner image formed on a photoconductor drum is transferred to a transfer material, and then the toner image fused by a fixing apparatus including a heating roller and a pressing roller is fixed to the transfer material.
In recent years, as a heating method of heating a heating roller, it is known that a heating member made of a heat-resistant film material having a thin metal layer (a conductor film) with a small heat capacity is shaped like an endless belt or a cylinder (a roller) and brought into contact with a fixing material by using induction heating. Compared with a heating method using a lamp or the like, in this method, the response to temperature changes in the heating roller is improved, the temperature is increased rapidly, and the warm-up time is reduced.
It is also known that an induction heater using induction heating includes a plurality of coils arranged in the length direction of a heating roller, and heats a predetermined area of the heating roller selected according to the size and other conditions of a fixing paper sheet. The induction heater flows a high-frequency current in a plurality of coils to generate an electromagnetic wave, and flows the induced current caused by the electromagnetic wave to heat a heating roller by the Joule heat generated by the induced current. By controlling the frequency of the high-frequency current flowing in this coil, the surface temperature of the heating roller can be changed, and the heating roller can be heated to a preset temperature.
When a high-frequency current with a different frequency flows simultaneously in each coil, interference noise may be generated by the different operating frequencies of the coils.
As disclosed in Jpn Pat. Appln. KOKAI Publication No. 9-80951, a fixing apparatus which prevents noise by connecting a plurality of coils constituting an induction heating means in parallel to decrease the inductance of the coils, and making the oscillation frequency generated by a resonance condenser and coils out of an audible frequency band, has been known.
This fixing apparatus neither assumes the noise generated when each coil is independently driven, nor prevents interference noise generated when each coil is independently driven.
As disclosed in Jpn Pat. Appln. KOKAI Publication No. 2002-124369, it has been known that in an induction heater having a plurality of induction coils for heating a heating member by induction heating, a switching means for turning on/off the input to each induction coil, and an induction heating inverter by a control means for controlling the turning on/off of the switching means, the control means synchronizes with the switching operation to turn on/off the input to each induction coil, and obtains an on/off signal by pulse width modulation.
Further, as disclosed in Jpn Pat. Appln. KOKAI Publication No. 2002-34241, there is another known induction heater having a heating means including a plurality of coils, in which the control circuit of one channel controls and varies the turning on/off width, and the control circuit of the other channel effects control by using a control signal curtailed from the signal synchronized with the one channel, thereby preventing interference noise and reducing uneven heating.
The above induction heaters use a control circuit to keep the frequency of high-frequency current flowing in the coils constant. Thus, it requires double number of switching elements compared with a circuit using a semi-E class self-excited system. Therefore, the thermal loss of the switching elements is doubled, and the efficiency is decreased. Concretely, if the thermal loss of one switching element is about 4%, the thermal loss by the switching elements become about 8% in the disclosed induction heater, and the heating efficiency is bad compared with a circuit using a self-excited system.
Further, the disclosed induction heaters use a circuit having a constant frequency, and when a pulse is forcibly given, the load on an inverter is increased, and a current value may be exceeded or the largeness of power may become difficult to control.
In the curtailed control disclosed in the Jpn Pat. Appln. KOKAI Publication No. 2002-34241, when one frequency is lowered, the frequency of a coil itself may become an audible frequency range, and noise may be generated.
Further, as disclosed in Jpn Pat. Appln. KOKAI Publication No. 2004-20776, a fixing apparatus having an induction heating means for heating a heating roller and a pressing roller has been known.
This fixing apparatus has a circuit to control an induction heating means for heating a heating roller, and a circuit to control an induction heating means for heating a pressing roller. These control circuits are independently operated. Therefore, the cost of circuits is increased compared with a fixing apparatus having only one circuit to control an induction heating means for heating a heating roller.